During certain semiconductor processing steps, the wafer can become adversely charged as to threaten the integrity of the semiconductor devices being fabricated thereon. For example, in the ion implantation process, a beam of positive impurity ions is accelerated and directed onto the surfaces of semiconductor wafers. The wafers are typically positioned on a spinning wheel so that the wafers are periodically implanted and simultaneously charged by the high energy positive ions. Although the wheel is grounded, it is insufficient to remove all or a substantial portion of the positive charges accumulated on the wafers.
In order to neutralize the charging effect, a number of known apparatus and methods have been developed. One is the electron shower where primary electrons are thermionically emitted from a filament. The primary electrons strike a target which emits secondary electrons toward the wafers. In another known method using thermionic emissions from a filament, the primary electrons strike a target which emits secondary electrons through a gas to create a weak plasma. The weak plasma is the source of electrons that may be drawn toward the wafers to counteract positive charging.
In both electron shower schemes, the high energy primary electrons tend to reach the wafers in excess of the number needed to neutralize the positive charge, thus overcharging the wafers negatively. The primary electrons also tend to charge the surface of the target which causes the secondary electrons emitted therefrom to be charged at the same energy as the charged target. The presence of primary electrons also induces a space charge effect in the path of the electrons which tends to divert the primary electrons from the target. Efforts to filter the primary electrons are complicated and difficult to implement due to the unpredictable nature of the charging effects of the wafers. Therefore, a primary electron filtering scheme must be tailored to each individual ion implantation equipment and process. Further, the filament electron source is fragile and prone to breakage if not handled or operated properly.
A third known method for neutralizing the charged wafers is a plasma flood system which creates a plasma that is contained electrically and magnetically near the wafer. The electrons in the plasma are pulled to the wafer to counteract the positive charges thereon. One problem with the plasma flood system is the complex and sensitive nature of the plasma containment system which is difficult to control to achieve good results. Further, the filament used to generate the electrons in a plasma flood system often experiences reduced operating life due to the introduction of a variety of insulators in the containment system that tend to coat the filament.